The present invention relates to a semiconductor integrated circuit having electrically erasable and programmable nonvolatile memory elements. More particularly, the invention relates, for example, to techniques which are effective when applied to a semiconductor integrated circuit having a nonvolatile memory wherein two nonvolatile memory elements are used as a storage unit.
In recent years, as a memory device in which data or program-constituting data are stored, substantial public attention has been directed to a flash EEPROM (hereinbelow, termed “flash memory”), which is a nonvolatile storage device from/into which stored data/data to be stored are electrically erasable/programmable collectively in predetermined units. The flash memory has its memory cells configured of electrically erasable and programmable nonvolatile memory elements, and it is capable of erasing data or program-constituting data once written into the memory cells and rewriting (programming) new data or program-constituting data into the memory cells.
Therefore, for the purpose of, e.g., altering data, correcting the bugs of a program or updating a program after a flash memory or a macrocomputer having a built-in flash memory has been assembled into an application system, data or data constituting the program as stored in the flash memory can be altered, so that the term necessary for the development of the application system can be shortened, and so that the flexibility of the development of the program of the application system is enhanced.
On the other hand, in recent years, note has also been taken of a system semiconductor device (hereinbelow, also termed “system LSI”) wherein one system can be constructed of a single semiconductor integrated circuit device by forming on a single semiconductor substrate a central processing unit (hereinbelow, also termed “CPU”) as a data control device, a dynamic random access memory (hereinbelow, also termed “DRAM”) as a large-scale storage device, a static random access memory (hereinbelow, also termed “SRAM”) as a high-speed storage device or cache memory, and other functional circuits. Such a system LSI is effective for reducing the size of a printed circuit board or packaging circuit board, etc., and especially for reducing the size and lightening the weight of a portable telephone set, a portable data terminal, and similar portable equipment.
Incidentally, after the completion of the present invention, the inventors investigated into known examples from a viewpoint-A and a viewpoint-B, as stated below.
The viewpoint-A concerns the use of a polysilicon gate of single layer for forming the memory cell of a nonvolatile memory, while the viewpoint-B concerns the use of two memory cells in a differential fashion.
As a result, regarding the viewpoint-A, there have been found the official gazette of U.S. Pat. No. 5,440,159, the official gazette of U.S. Pat. No. 5,504,706, the official gazette of Japanese Patent Application Laid-open No. 212471/1992 (the official gazette of corresponding U.S. Pat. No. 5,457,335), and Oosaki et al., “A single Ploy EEPROM Cell Structure for Use in Standard CMOS Processes”, IEEE Journal of solid sate circuits”, VOL. 29, NO. 3, March 1994, pp 311-316.
On the other hand, regarding the viewpoint-B, there have been found the official gazettes of Japanese Patent Applications Laid-open No. 163797/1992, No. 263999/1989, No. 74392/1992, No. 127478/1992, No. 129091/1992 and No. 268180/1994, and the official gazette of U.S. Pat. No. 5,029,131.
By the way, the official gazette of Japanese Patent Application Laid-open No. 212471/1992 discloses also a technique which utilizes an electrically programmable nonvolatile memory (EPROM) as a remedy circuit for a read only memory (ROM). Further, the official gazette contains the statement that the nonvolatile memory element of single-layer gate structure according to this invention can be utilized also as an electrically programmable and erasable nonvolatile memory element which executes programming with hot carriers and executes erasing with a tunneling current by applying a high voltage to a source or a drain, or which executes programming and erasing with tunneling currents.